The most common fabrication technology for SCRF cavities is to form thin walled (e.g., about 1 to 3 mm) shell components from high purity niobium sheets by stamping. These shell components are welded together to form hollow cavities. Horizontal processing of horizontally situated cavities as described in U.S. Pat. No. 4,014,765 (Siemens Corporation) was developed to avoid the adverse effects of gas pockets and bubble entrainment that lead to nonuniform electropolishing. A schematic illustration of an apparatus for conducting horizontal processing of SCRF cavities is shown in FIG. 10. The SCRF cavity 100 is oriented horizontally and mounted on a pair of rotatable end caps 120. One of the end caps includes a circumferential electrically conductive surface 140. A cathode 160 passes through the cavity 100. In the illustrated embodiment, the SCRF cavity includes a single cell which is schematically represented by the large diameter portion in the middle of the body shown in the figure. The cathode 160 is electrically connected to a rectifier 400 by the cathode lead 440. The anode lead 420 of the rectifier 400 is connected to the rotating conductive surface 140 which is electrically connected to the SCRF cavity 100. The cavity 100 is partially filled with a viscous electrolyte 320. The electrolyte 320 is supplied from a tank 300 via the electrolyte feed tube 340 which dispenses the electrolyte to the cavity 100. The electrolyte is continuously circulated through the cell 100. It leaves the cell through a return tube 360. The volume above the electrolyte 320 in the cell 100 contains gas generated during the electropolishing process. This gas is purged from this space by means of a vent shown schematically at 220. The gas purge 200 is introduced at the end cap 120A on the opposite end of the cell 100. The cavity is rotated on the end blocks 120 as shown by the directional arrow A in the figure.
One of the vehicles that is often used in electropolishing passivating metals like niobium is hydrofluoric acid. As explained herein, the electrolytes used with these passivating metals tend to be highly viscous and this can leading to the gas entrainment difficulties that have required the use of the horizontal processing design discussed above. Accordingly, there is a need for a method for polishing niobium and other strongly passivating metals, particularly for use in surface finishing SCRF cavities, that does not require the use of highly viscous electrolytes.
As explained in detail in U.S. Published Application No. 2011/0303553 to Inman electrochemical polishing or electrolytic polishing or electropolishing is a process whereby metal (M0) is selectivity removed from a surface by an electrochemical reaction, generally of the formM0→Mn+ne−  Eq. 1
As illustrated in FIG. 1, during electropolishing, the current distribution is controlled so that the peaks or asperities of the surface are preferentially removed relative to the recesses or valleys in the subject surface. In the case of primary or geometric current distribution as depicted in FIG. 2, the resistive path length from the cathode to the surface asperity (Ωp) is shorter than the distance from the cathode to the recess (Ωr). Consequently, the peaks are preferentially dissolved. The difference in the current distribution between the peak and recess is greater as the electrolyte resistance increases. Highly resistive electrolytes and low electrolyte temperatures are desirous to increase the differential between the current at the peak and the recess. Decreasing temperature increases resistivity.
In the case of tertiary or mass transport controlled current distribution as depicted in FIG. 3, the diffusion distance from the peak to the bulk solution (Dp) is less than the diffusion distance from the recess to the bulk solution (Dr). Since one skilled in the art would understand that the diffusion limited current is based on either dissolved metal ions diffusing away from the peaks or acceptor ions diffusing to the peaks, the diffusion limiting current for metal dissolution at the peaks is higher than the diffusion limiting current at the recesses. Consequently the peaks are preferentially dissolved. The difference in the diffusion limited current distribution between the peaks and recesses is higher for viscous solutions. Viscous solutions have the effect of slowing down the diffusion process. Consequently, highly viscous electrolytes (e.g., about 15 to 30 cP) and low temperatures (e.g., 10° C. to 30° C.) leading to higher viscosity are able to increase the differential between the current at the peak and the recess. Consequently, electropolishing solutions used in the systems discussed above are generally highly resistive (e.g., 10 mS/cm to 200 mS/cm) and high viscosity (e.g., about 15 cP to 30 cP) solutions, in some cases operating at low temperatures as disclosed by D. Ward “Electropolishing” in Electroplating Handbook ed. L. Durney 4th edition pg. 108, Van Nostrand Reinhold, N.Y. (1984).
Despite the obstacles presented by strongly-bonded passivation layers, various techniques have been developed for electrochemically processing such metals as niobium and niobium alloys. In addition to highly resistive and high viscosity electrolytes, these techniques typically require high voltages and/or hydrofluoric acid in the electrolyte solution. The electrochemical conditions which drive the reaction shown in Eq. 1 above also drive the following reaction which results in the formation of passivating oxides.M0+xH2O→MOx+2xH++2xe−  Eq. 2By electropolishing in non-aqueous or minimally aqueous electrolytes, the source of the oxygen that forms these passivating oxides is eliminated. However, maintaining low water content presents an additional set of control challenges. Using reverse current pulse conditions in accordance with this disclosure provides the means to manage the formation of this layer of passivating oxides, even in the presence of substantial water, so that the oxides do not interfere with electropolishing.